How-To: Fourier Transform Infrared Spectroscopy (FTIR) Jacob Grant

Professor Scott Shaw Research Group

Group Meeting 141027

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Beer-Lambert Law

T = transmittance (relative amount of light passing through a sample)

I, I0 = intensity (Wm-2) of transmitted and incident radiation, respectively

𝜀 = molar absorptivity (m2mol-1 or M-1cm-1) or how strongly a chemical species absorbs light at a given wavelength

b = path length (distance light travels through the chemical species)

c = molar concentration of chemical species

A = absorbance (relative amount of light absorbed by sample)

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𝑇 =𝐼

𝐼0= 𝑒−𝜀𝑏𝑐

𝐴 = −𝑙𝑛 𝑇 = −𝑙𝑛𝐼

𝐼0

𝐴 = 𝜀𝑏𝑐

• Absorbance is determined in the IR spectrum, and concentration should be known for a liquid sample • Spacers of accurately determined thickness can be used for path length, and molar absorptivity can then be calculated • If molar absorptivity and concentration can be determined, the thickness of a film can be calculated by solving for the

path length

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Coherent Light

Source

Beam Splitter

Stationary Mirror

Moving Mirror

Theory

Sample

Detector

Recombined Beam

Michelson Interferometer • Moving mirror varies wave interference, and

the sample is exposed to light of a wide range of wavelengths

• Quickly acquire data in a wide spectral range

FFT

Frequency Domain Spectrum

• Molecules absorb specific frequencies related to their structure

• Qualitative and quantitative functional group identification

Time or Spatial Domain Interferogram

Sample Preparation

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KBr Salt Plates • Handled via edges (fragile) with gloves • No fingerprints or water exposure, clean

copiously with ethanol or acetone • Collect a background spectrum with clean

KBr plates secured in sample holder • Place a few drops of sample solution on

one salt plate, press the plates together, and collect a sample spectrum

• Use a (clean) spacer between the salt plates with sample when the path length must be known

Gas Cell

Salt Plate and Sample Holders

Sample Chamber

Operation

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Software Operation • Must cool detector with liquid nitrogen • Software – “Omnic”

• Instrument initializes and attains stable interferogram

• Instrument will indicate all tests passed • Save Data – “IR Group Data” • Experiment – “Transmission E.S.P.” • Set Parameters – “Expt Set”

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Expt Set – Collect No. Scans – 128 Resolution – 2 Final Format – Absorbance Typically use specified background file

Expt Set – Bench Sample Compartment – Main Detector – MCT/A Gain – 1.0 Optical Velocity – 1.8988 Peak to Peak - ~7-8

Aperture and Attenuation (control light intensity reaching detector) – alter to give peak to peak of 7-8

RECORD ALL PARAMETERS

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After parameters are set • Click ok, place your background in the

chamber, and click Col Bkg • Save this file and label it clearly as a

background spectrum • Return to Expt Set > Collect and use the

specified background file under “Background Handling”

Bare silicon wafer spectrum

Use the same background file within a set of runs, but collect a new background when the experiment is repeated on a different day

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Sample Spectrum • After background has been collected and

identified in Expt Set, place your sample in the chamber and click “Col Smp”

• You should not be prompted to collect a background

• Save sample spectrum first as a spectra (.spa) file and then as a csv text (.csv) file for later use in Excel/Origin

Organic Film on Silicon Wafer

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Baseline a Spectrum • Process > Baseline Correct… • Place points along raw spectrum baseline

to create a flat baseline • Place points away from peaks to avoid

affecting peak shape • Click Replace • Save processed spectrum as a new file, do

not overwrite raw data

Further Processing With the CSV files, spectra can be opened in Origin for thorough peak fitting and analysis

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